Workers

Elm has Platform.worker programs – programs without view. They are sometimes used in Web Workers or in Node.js.

elm-watch works in both Web Workers and Node.js. The biggest difference compared to running in a web page, is that you don’t get the Browser UI. Instead, elm-watch only logs to the console. (There is currently no UI to switch to optimize mode (worker programs don’t have a debug mode).)

Another important difference, is that outside a web page there is no concept of “reloading the page”. Sometimes it is not possible to hot reload an Elm app, such as when adding a new port. Then the whole application needs to be reloaded. In a web page, elm-watch automatically reloads the page. In a Web Worker or Node.js, elm-watch can’t do that. Instead, elm-watch logs a message about this, and you need to restart your program yourself. Luckily, there is a way you can improve this experience.

You can define globalThis.ELM_WATCH_FULL_RELOAD to be a function that does a “full reload”:

globalThis.ELM_WATCH_FULL_RELOAD = () => {
  // Do whatever you need to restart your application here.
};

For an example of how to do this in Node.js, see example/nodejs-program.js. It’s not particularly easy, but it is possible!

Note that you need a recent enough Node.js version that supports the WebSocket global (in short, basically Node.js 22 or later).